[S-CG40] Active Tectonics and Seismic Hazards in the Himalayan Region

[SCG40-08] Microtremor observation in the Kathmandu valley, Nepal: Past, Present, and Future perspectives

*Mukunda Bhattarai¹, Rajesh Sharma³, Sunita Bhattarai², Tara Pokrarel¹, Chintan Timsina¹, Suresh Shrestha², Dinesh Nepali², Manoj Nakarmi⁴, Prakirna Tuladhar⁴, Takumi Hayashida⁵, Toshiaki Yokoi⁵, Hiroe Miyake⁶, Koichi Kusunoki⁶ (1.National Earthquake Monitoring and Research Center, Department of Mines and Geology, Nepal, 2.Department of Mines and Geology, Nepal, 3.Seismological Center Surkhet, Department of Mines and Geology, Nepal, 4.Department of Urban Development and Building Construction, Nepal, 5.International Institute of Seismology and Earthquake Engineering, Building Research Institute,Japan, 6.Earthquake Research Institute, The University of Tokyo, Japan)

Keywords:Array deployment, Kathmandu valley, Shear wave velocity structure, Gorkha earthquake, RC buildings

Kathmandu valley falls in the Lesser Himalaya of central Nepal and is one of the largest intermontane basins of Nepal. It is prone to earthquake disasters due to its tectonic environment and local site conditions. Furthermore, accumulated piles of thick sediment above the basement rock, steep topography in basin outskirts, unplanned land use/settlements, rapid urbanization, and poor earthquake-resistant constructions have made this region highly vulnerable to earthquake disaster. These factors emphasize the need for subsurface structure studies and their applications to damage evaluation of Reinforced Concrete (RC) buildings. Not only the evaluation of site-specific seismic hazard where the dwelling is located using passive surface waves but also surveys inside the building itself is of great concern for the evaluation of future seismic risk, where about 3 million agglomerates are residing.

Passive microtremor measurement has been performed in the Kathmandu valley before the Gorkha earthquake using a single tri-axial seismometer (Pandey et al., 1996, Paudel et al., 2010, Bhandary et al, 2014). After the Gorkha earthquake, there exist few studies in the Kathmandu valley for the site-specific studies using microtremor (JICA, 2016, Poovarodom et al., 2017, Molnar et al., 2017). These studies also have the limitation of revealing the deep shear wave velocity structures up to the basin basement.

The project for “Integrated Research on Great Earthquakes and Disaster Mitigation in Nepal Himalaya” is a multidisciplinary research project (FY:2016-2021) in Nepal supported by Japan Science and Technology Agency (JST) and Japan International Cooperation Agency (JICA). The main objective of this project is to provide an advanced seismic risk assessment of the Kathmandu valley by subsurface explorations and establishing seismic observation networks targeting around the central seismic gap along the Main Himalayan Thrust (MHT) to contribute to disaster risk reduction of Nepal by utilizing data acquired from explorations/observations and their research findings.

There are five working groups in this project, group 3 (G3) where most of the authors belong is mainly involved in subsurface explorations of the Kathmandu valley, whereas group 5 (G5) which reflects the overall output of the project is mainly based on inputs from G1 to G4. Few outcomes of microtremor explorations have already been documented for e.g., Bhattarai et al., 2017a; 2017b, Yokoi et al., 2018; 19, Hayashida et al.,2018; 2019. However, most of these studies could not obtain the dispersion curve and consequently velocity structures in the frequency range from 0.1 to 0.2 Hz.

In this meeting, I will discuss the period verification of Nepalese RC buildings revealed from microtremor observations in some sampled RC buildings with the building codes and explain the application of shear wave velocity structures to building damage analysis as a potential output of (G5) and also emphasizes its importance on the future disaster mitigation of the Kathmandu valley.

Acknowledgments:

This research was supported by Science and Technology Research Partnership for Sustainable Development (SATREPS) from Japan Science and Technology Agency (JST) and Japan International Cooperation Agency (JICA). I highly acknowledge the Director General of the Department of Mines and Geology (DMG) for providing me an opportunity to join this scientific gathering remotely.